Two Studies of George Dyer with Dog by Francis Bacon. Photograph: Alamy Stock Photo

Joule said Bacon knew the drugs belonged to his lover. Dyer kept them in a large hole in a statue given to him by his gangster associates the Kray twins.

Joule said that when they discussed the incident Bacon pointed to a photograph of a masterpiece titled Two Studies of George Dyer with Dog 1968, which is now in a private collection.

Bacon says on the recording: “At the time of my arrest, and for a long time after, I stayed furious. To release some of the tension, I straight away went to my easel … did a large painting. He [Dyer] is sitting on a chair with the nasty flattened police dog at his feet sniffing towards the statue … which became George’s head.”

The artist explains that he first depicted the statue on a table at the front of the painting, but changed it to show Dyer’s head.

Joule said that although Bacon had argued in court that he would never break the law, the tapes revealed he had known the drugs were there and that they had not been planted.

In the recording, Bacon speaks of the African sculpture, which was a birthday present from Dyer. “He [Dyer] said the Krays gave it to him. I’m sure they did … I never smoked any dope as I have asthma, but George certainly did when he came here,” he says.

“So, after giving it to me, then scraping the base covering away … he pulled the stuff wrapped in a thin foil out of the hollowed-out statue … Then one day after we’d had a series of terrible rows in a great rage he reported me to the Chelsea drug squad. They raided the place here and their trained labrador sniffed out the drugs in the statue. It was all a dreadful mess … but in the end Goodie [Arnold Goodman, a solicitor] defended me and I got off … finally paying a huge amount in costs. The judge didn’t like me much.”

Although Bacon later claimed Dyer had planted the drugs, he had known all along that Dyer’s cannabis was in his studio and had “seemed little bothered by it until that shocking revenge day”, Joule said.

Joule lived 20 metres from Bacon’s studio and home in South Kensington, and their friendship lasted from 1978 until the artist’s death in 1992.

Bacon, whose 1969 portrait Three Studies of Lucian Freud sold in 2013 for a record £89m, agreed to be recorded on condition that Joule did not release the tapes until at least 12 years after his death.

In 2004 Joule donated to the Tate 1,200 sketches from Bacon’s studio, then valued at an estimated £20m and described as one of the most generous gifts to the gallery.

Joule kept about 120 drawings. He is now lending about 70 to an exhibition in Italy, at the Fondazione Sorrento Museo in Sorrento between 19 May and 21 October. Bacon gave Joule the African sculpture, which he will show for the first time.